This research program describes experiments to be performed with sets of T helper cells defined in terms of their antigen specificity: 1) hapten specific T helper cells, with receptors bearing Ig-idiotype like determinants; 2) T helper cells which recognize specifically immunoglobulin idiotypes and can trigger B cells bearing these idiotypes in the absence of conventional soluble antigens; and 3) "polyclonal activator T helper cells" which recognize B cell membrane antigens. T helper cells will be highly enriched in long term cultures, depleted of alloreactivity, and titrated in T-B collaboration assays in vitro. We will study the "T cell idiotype" in the context of the functional MHC restriction of T helper cells. We will investigate the relation between T cell genotype and thymus environment in the establishment of predominant T cell clonotypes. We will try to determine the role played by network type of interactions between sets of T helper cells and B cells in the selective triggering of B cells. We will try to establish and clone a line of PC specific T helper cells, which bear the TEPC 15 idiotype and a clone of T helper cells which specifically recognize the T15 idiotype.